Multistep Reaction Energy Profile (College Board AP® Chemistry): Study Guide

Multistep reaction energy profile

• The energy changes that occur in reactions involving a multistep mechanism can be shown in a graph

• A multistep energy profile can be used to:

  • Identify the number of elementary reactions

  • Determine (or calculate) the activation energy of each step

    • Consequently, identifying the rate-limiting step

  • Identify the number of intermediates involved in the reaction

  • Determine (or calculate) the overall energy change or enthalpy change

• For example, consider a hypothetical reaction given by the equation:

A + 2B → D ΔH > 0 (endothermic)

• If the proposed mechanism for this reaction involves a two-step elementary reaction given as:

A + B ⇋ C (fast)

C + B → D (slow)

• To sketch the profile from the mechanism:

  • Draw one hump for each elementary step

  • Give the slow (rate-limiting) step the largest activation energy and the fast step a small one

  • Put each intermediate in a trough between two humps

  • End the products above the reactants if ΔH is positive (endothermic) and below them if ΔH is negative (exothermic)

• The energy profile for this reaction may look like this:

Multistep energy profile

An energy profile diagram for a two-step reaction showing the activation energy, overall enthalpy change, and the intermediate

• From the graph above:

  • The number of steps involved in the reaction is indicated by the number of peaks

    • Each peak also represents an activated complex/transition state

  • The step with the largest activation energy (the biggest rise from a trough to the next peak) is the rate-limiting step

  • The number of intermediates is the number of troughs between peaks

  • The overall enthalpy change, ΔH, is indicated by the difference in energy between the products and the reactant